Catheter shafts are generally made of flexible, bio-compatible material, such as plastic, including polyethelene, polyester or polyimide. However, advancement into and through a patient's body and manipulation of a catheter can be difficult if the catheter shaft lacks a certain amount of stiffness. Moreover, it can be particularly challenging to provide suitable flexibility, particularly near the shaft tip, while maintaining adequate overall shaft stiffness. While there are devices available to provide or add stiffness to catheters, including stiffener wires, they are typically separate and not integrated in the catheter such that separate handling and manipulation are required. Indeed, there are few, if any, known mechanism internal to the catheter that can advance stiffening wires or other devices into, or through, EP catheters. It is therefore desirable to provide a catheter having an internal advancing mechanism that can advance stiffening wires or other devices into or through the catheter shaft.